1. Field of the Invention
This invention relates to carboxyl-containing polymers (such as polyacrylic acid) which when neutralized are useful as thickeners and emulsifiers for aqueous mixtures. More specifically, this invention relates to an improved process for neutralizing acrylic acid copolymers and crosslinked acrylic acid copolymers to yield a thickening agent comprising the polymer suspended in an organic solvent and characterized as containing a higher percentage of polymer solids than previously achieved for such thickening agents.
2. Description of the Prior Art
It is known that copolymers of carboxyl-containing monomers, such as acrylic acid and about 0.1 to 1.0 by weight of a crosslinking monomer, such as polyallyl sucrose, are gel-like polymers which are insoluble in water and organic solvents and which, especially in the form of their salts, absorb large quantities of water or solvent which causes the copolymer to increase many times in volume. These polymers, because of their swelling properties, are useful as bodying and suspending agents in various mucilaginous and colloidal gel-like applications such as dentifraces, surgical jellies, creams and ointments, printing paste thickeners, and the like.
It is the neutralized salt (most likely a partial alkali, ammonium or amine salt) of the copolymer which exhibits maximum thickening or emulsifying properties. Typically, the neutralized salt is prepared by combining a slurry of the copolymer and several surfactants in a solvent with a neutralizing agent such as ammonium hydroxide. The neutralizing reaction is highly exothermic. If the polymer content of the slurry to be neutralized is too great, removing heat from the neutralization reaction is difficult, and the polymer has a tendency to gel which makes it difficult to transfer the neutralized product from the reactor.
In its useable form, the neutralized polymer, in the presence of several surfactants is diluted by and is suspended in an organic solvent. Typically, such mixtures contain 20 to 35 wt percent of the polymer which is the maximum routinely achieveable without a runaway exothermic reaction or gelling during the neutralization. Further, these mixtures are very effective thickening agents when combined with aqueous solutions. For example, only a tablespoon of such mixtures is required to thicken several gallons of aqueous solutions. However, if the polymer content of the mixture could be increased, then even less of the thickening agent would be required to produce the same amount or degree of thickening.
An objective of the present invention is to develop a process for the neutralization of crosslinked copolymers of carboxyl-containing monomers and the production of thickening agents containing such polymers. This improved process which eliminates or controls the high reactor temperatures resulting from the exothermic neutralization reaction, produces no gelling of the copolymer, and results in a thickening agent having a high weight percent polymer solids.